This invention relates to a method and apparatus for dehydrating sludge such as sewage sludge.
It is well known that sludge produced in sewage treatmentplants are fed to a belt screen press for dehydration after organic and/or inorganic additives are added thereto to accelerate the coagulation thereof.
In a well-known belt screen press, sludge is fed on the portion of a water-permeable endless belt which is arranged on a horizontal plane by being trained around a plurality of guide rolls and which is not subjected to pressure and are dehydrated by gravity acting thereon. The sludge is then carried between said endless belt and another endless belt trained around another set of guide rolls to be guided to run in contiguous relation with the first mentioned endless belt. The sludge so carried between the two endless belts is compressed betwen one or more pairs of compression rolls which are disposed in opposed relation so as to permit the endless belts to pass between them in immediate contact relation therewith. In another well-known method, sludge is compressed between a drum which is supported in suitable bearings, with its periphery in immediate contact with one of the two endless belts, and a pressure belt which is trained around aplurality of rolls, with its surface in immediate contact with the other of the two endless belts, compression pressure being exerted over the portion of the two endless belts carrying sludges therebetween which surrounds the drum interposedly between the drum and the pressure belt.
This invention relates more specifically to the latter mentioned method, or more concretely, a method wherein sludge is fed into the first section of a path of travel of two endless belts, each trained around a plurality of guide rolls, and which are caused to run along said path of travel in contiguous relation with each other, at least one of said two endless belts being made of water permeable filter medium, and said sludge, carried between said two endless belts, is compressed to be dehydrated by and between a drum which is supported in suitable bearings, with its periphery in immediate contact with one of the two endless belts, and a pressure belt which is trained around a plurality of rolls, with its surface in immediate contact with the other of the two endless belts, and an improvement thereof.
Patent disclosures dealing with dehydrating apparatuses of this type are found in Japanese Patent Application Disclosure No. 123964 of 1976 and Japanese Utility Model Application Disclosure No. 104172 of 1977.
In these dehydrators of belt press type, the pressure belt which is in immediate contact with one of the endless belts exerts pressure upon at least one of the endless belts against the drum, as it is tensioned, so that the sludge carried between the endless belts is heavily compressed by the pressing pressure caused between the endless belt and the drum and not through the tension of the endless belts. Therefore, the water-permeable endless belts between which the sludge is carried need not be subjected to any unreasonable tension. This is an advantage in that the seam of the endless belt is less likely to break due to tension. Another advantage of dehydrators of this type is that the sludge-carrying portion of the endless belt undergoes surface compression in the course of its passage between the pressure belt and the drum, whereby a high dehydration rate is assured.
As disclosed in Japanese Patent Application Disclosure No. 128964 of 1976, a pressure belt composed of multiple belt strips is found to be advantageous: a multiplicity of belt strips pressing the outer water-permeable webs which surround the drum are traversely spaced apart from one another. This arrangement shows good dehydration results.
However, in these prior art apparatuses, consideration is not given as to how to apply surface pressure. Where the run of the pressure belt around the drum is small, e.g., 90.degree. or less in terms of angle of contact, pressure loading toward drum surface due to the tension of the pressure belt is not so variable because curvature between the beginning point and the ending point of contact is small. In this case, no problem is involved. On the other hand, if the run of the pressure belt around the drum is 180.degree. or more, for example, in terms of angle of contact, pressure loading differs substantially between the beginning or ending points (smallest) and the middle contact point (largest) because curvature is large. This means that the pressure belt serves substantially for dehydration during the first half of its run but does not function well during the latter half of its run.
In conventional press-type dehydrators, the pressure belt is in contact with the sludge-carrying endless belts, with the same pressing coverage throughout its run. As soon as sludge enters the pressing zone, abrupt compression pressure is exerted thereupon, and consequently mesh clogging is often caused to water permeable endless belts at an early stage of pressing operation. Thus, the water penetrability is reduced and the subsequent dehydrating operation does not progress smoothly for the amount of pressure effected.
Such problem involved in prior art belt press filters has not been known. A solution to this problem is also required. Therefore, it is an object of this invention to provide a method and apparatus for smoother and more efficient dehydrating operation.
More specifically, it is proposed that in a method for sludge dehydration using belt press type apparatus as mentioned above, a pressure belt is composed of a plurality of belt strips, the path of travel of the endless belts around the drum being divided into a plurality of stages, pressing of sludge by pressure belts being applied at traversely spaced positions, stage by stage, by alternating and/or increasing such positions.